vfs: handle _auditpath in proxyvfs Just forward the call to the underlying vfs.

vfs: fix proxyvfs inheritance The proxyvfs class is designed to overwrite some of the vfs logic. Yet, it did not use normal class inheritance. This is becoming an issue as `abstractvfs` method could take precedence over their `proxyvfs` version. We fix the inheritance chain to be as expected.

vfs: also audit rename Renaming through the vfs is not used in many places, and none of them seems to be a security risk. However, it is still worthwhile to run the auditing on rename file to perform developer-warning level checks.

vfs: add a `_auditpath` to abstract vfs We are about to make `rename` audit path. Since rename lives in the `abstractvfs` layer, we need it to be aware of auditing to some extent. The default implementation is no-op because multiple existing vfs are not using auditing at all right now (eg: fncachevfs).

vfs: makes all audited path relative Only auditing relative path helps the vfs warning logic.

debugupgraderepo: add a --no-backup mode The process has been around for a while and is pretty safe now. Having an automated way to clean up the old data is useful when running many different conversion to compare delta algorithm.

upgrade: add '-' in optimization name The older name `redeltaall` was hard to type and read. The newer form should be more user-friendly. We keep backward compatibility with the old form (at least for a while). Having to use different form depending on the version is very impractical and error prone.

revlog: add test case for _findsnapshots Testing the method directly is more robust.

revlog: use the native implementation of issnapshot In some sparserevlog case where a lot of the history has to be searched for a snapshot, the cost of issnashot cost becomes significant. The computation done by the method is fairly low level, a native implementation provide a very significant speedup. example affected manifest write before: 0.490375s after: 0.114989s (-76%)

revlog: add a native implementation of issnapshot This will be used in the next changesets